Akihiko SUZUKI1 and Shin KUBOTA2 : Traces of the predatory
gastropod (ichnospecies Oichnus simplex) found in shells of
Spondylus sp. (Mollusca, Spondylidae) washed
ashore at Shirahama Town, Wakayama Prefecture, Japan ~7KBA~ 1 •
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Introduction
Molluscan shells, such as those belonging to the ichnogenus
Oichnus Bromley, 1981, are frequently characterized
by the presence of circular holes, which are traces produced by
predatory gastropods. Predator-prey relationships have
been studied extensively in the modem benthic realm (Carter
1968; Carriker 1981; Kabat 1990) and in the fossil record
(Taylor 1970; Kelley 1988; Kowalewski et al. 1998), and among
the most extensively studied indicators of biotic inter-
actions are predatory drilling holes, as these provide
considerable information on predator-prey relationships
(Vermeij
1987; Kelley 1988; Kowalewski et al. 1998).
In July to August 2015, predated shells of the spondylid bivalve
Spondylus sp. were collected at Shirahama Town
in Wakayama Prefecture (Kubota 2015). Here we describe the drill
holes produced by predatory gastropods in the
shells of S. sp., and discuss the ichnology of drill holes and
other aspects of predator-prey relationships.
Materials and methods
More than 600 specimens of S. sp. were collected on Kitahama
beach at the Seto Marine Biological Laboratory of Kyoto
University
in Shirahama Town in Wakayama Prefecture (Kubota 2015). Of
these specimens, 14 with drill holes and three entier specimens
were
subjected to morphometric analysis. Measurements of shell
height
were performed using digital calipers, and pictures of drill
holes in
S. sp. shells were captured under a stereomicroscope.
Results and discussion
Circular to subcircular drill holes that completely penetrated
the
shells of the smaller bivalve shells are shown in Fig. 1. The
holes
Fig.1 Hole patterns on the left shells of Spondylus sp. found on
Kitahama beach at the Seto Marine Biological Laboratory, Kyoto
University.
bored through the shells appeared to be
randomly distributed.
Specimen A, shell height 64.49mm,
had a hole with a diameter of 3.35mm
(Fig. 2A). Specimen B, shell height
63.37mm, had a hole w.ith a diameter of
3.42mm (Fig. 2B). Specimen C, shell
height 50. 65mm, had a hole with a
diameter of 2.54mm (Fig. 3). Circular
holes measured ca. 2.5 to 3.5mm in
diameter. When the hole of specimen C
was cut to produce a transverse section
(Fig. 3A), the depth of the hole was
1.92mm and the depth-to diameter ratio
was
(Fig. 1). This is because the right valves of S. sp. were
attached to the substrate, leaving the left valves exposed to the
environment and susceptible to attack by predators. We found that
only small S. sp. shells had holes, and that larger shells did not;
it was considered that this was because the large shells were too
thick to for predators to penetrate. Further, the positions of the
holes on the shell appeared to be random and no obvious patterns
were recognized (Fig. 1).
In transverse section, the holes were slightly conical and
perpendicular to the shell surface. The walls of the holes appeared
to be smooth. Based on the cylindrical morphology of the drilled
holes, the predators likely belonged to the ichnotaxon, Oichnus
simplex Bromley, 1981. This type of predation is widespread among
muricid whelks (Bromley 1981; Kabat 1990), which are common in
shallow seas where they prey on epifunal mollusks (Kabat 1990;
Ishida 2004).
Numerous muricid whelks (32 species in 20 genera) have been
reported in the Shirahama area (Kubota and Koyama 2002). Among
these species, shallow water species were most likely to be hole
borers. Future research will identify the potential gastropod prey
of these muricid whelks.
Acknowledgments
This study was supported by a Grant-in-Aid for Scientific
Research from the Japan Society for the Promotion of Science ( (C)
16K01002).
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